At present, attention has been paid to positron emission tomography (PET) as a technique that is high in sensitivity to be excellent in quantitatively determining performance and can form images easily in light of a principle thereof. This technique has widely been used. The half value period of PET diagnostic reagents (tracers) used for diagnoses is short, and the tracers are each administrated in a fine amount so that any living body is hardly exposed to radiation based thereon. Therefore, this inspecting method is a low invasive inspecting method, thus is greatly advantageous to PET. Furthermore, PET is highly sensitive even to tumors that are not easily determined by CT (computed tomography) or MRI (magnetic resonance imaging), and tumor tissues thereof can be evaluated according to images.
18F-labeled BPA, in which a 18F-fluorine atom is introduced into BPA, which is a boronated amino acid used as a boron reagent for BNCT (boron neutron capture therapy), was developed as a molecular probe for PET by Ishiwata in 1991 (Non-Patent Document 1). Thereafter, a PET inspection with the use of 18F-labeled BPA using the present probe has been an important technique for supporting BNCT. In other words, in clinical and research spots, a 18F-BPA PET image obtained by measuring a subject beforehand can give data on an internal accumulation distribution of BPA, the ratio of tumor tissues/normal tissues (the T/N ratio) and others. On the basis of these data, curative effects of BNCT can be beforehand assumed and then a research or therapeutic plan can be drawn up.
In Ishiwata's synthesis method, BPA is directly fluorinated to prepare 18F-labeled BPA, and 18F+ is used as an electrophilic reagent. From deuterium (D) and neon (Ne) accelerated by a cyclotron, 18F gas is prepared, and then passed through a column filled with sodium acetate to convert the gas to CH3COO−18F+. Thereafter, a solution of BPA in trifluoroacetic acid is bubbled by the introduction of this conversion-obtained compound into the solution. In this way, the synthesis of the target 18F-labeled BPA is attained.
As another method for synthesizing 18F-labeled BPA, Vahatalo et al. suggest a method in which such a conventional method is partially improved (Non-Patent Document 2). This method is a method of using H18F, which can be obtained in a larger quantity, to attain the synthesis via CH318F as an intermediate of 18F2. By causing CH3I to react with H18F, which is obtained through the radiation of protons to H218O [through 18O(p,n)18F reaction], CH318F is once synthesized. The resultant compound CH318F is discharged to disassociate its C—F bonds to prepare 18F2. This compound is used to synthesize 18F-labeled BPA, equivalently to Ishiwata's synthesis method.